When converting a digital audio signal or other type of band-limited digital signal into analog form, it is desirable to preserve a given frequency band of the signal content, while filtering out unwanted frequencies in other bands (typically higher in frequency than the desired band). For example, digital to analog conversion often inherently exhibits a low pass filtering effect, which removes out of band noise in the resulting output analog signal.
When the analog output of a digital to analog converter is fed to an amplifier, such as a class D amplifier, out of band noise can have a negative interaction with the particular amplification process, and can alias back into the desired band, corrupting the output signal and making it unpleasant to listen to or even unusable. Low-pass filtering for removing the out of band noise during a digital to analog conversion process may be performed through the use of a semi-digital finite impulse response (FIR) filter. The input to the filter may be a pulse density modulated (PDM) digital audio signal, and the semi-digital FIR filter uses the digital input values to switch its voltage mode or current steering mode configuration on and off to generate an analog output version of the digital input signal that is also inherently low-pass filtered. In a conventional design, the coefficients of the semi-digital FIR filter are computed using classical digital filter design techniques in which the filter is constrained to be a linear phase filter, which results in a symmetrical set of FIR filter coefficients being computed.